A. Field of the Invention
The invention relates to fluidized bed combustors.
B. Prior Art
Fluidized bed systems are commonly used in the chemical processing industry for a variety of applications such as facilitating chemical reactions between various constituents, and for drying, coating, and other processes. Similarly, they find frequent use in the petroleum industry for the catalytic cracking of hydrocarbons.
Fluidized bed systems are also used to generate heat by the combustion of coal and other solid fossil fuel to produce steam as an output product. If the combustion occurs at atmospheric pressure, these systems are referred to as atmospheric fluidized bed combustors (AFBC). Typically such systems utilize a single fluidized bed, commonly four to six feet in depth, and operating at fluidization velocities of from four to nine feet per second. Heat recovery from these systems is obtained through heat transfer to tubes imbedded in the fluidized bed itself and containing a heat transfer fluid such as water or steam, as well as from tubes positioned above the bed in the path of hot gasses emerging from the bed. While such systems offer promise, they require a significant capital expenditure per pound of output steam.
Present generation fluidized bed combustors accomodate combustion of a wide variety of fossil fuels (of varied fuel heating value, ash content, and sulfur content) with the potential for controlling sulfur oxide emissions and reducing NOx (nitrous oxide) generation. Sulfur oxide emissions are controlled by adding sorbents such as limestone or dolomite to the burning fuel mass. The sulfur oxide gases generated during combustion react with limestone to form calcium sulfate and with dolomite to form calcium and magnesium sulfate. Thus, sulfur is retained as a solid product and removed from the bed along with the bed ash material.
The fluid bed normally operates in a temperature range of between 1450.degree. F. and 1600.degree. F. NOx formed by atmospheric nitrogen fixation is not a dominant reaction path for NOx generation at these temperatures. Thus, the conversion of fuel bound nitrogen to NOx is the only significant reaction pathway leading to NOx formation within the fluid bed. Hydrocarbon emissions, primarily CO, may be controlled by adding sufficient excess air to the combustor. Ash and spent solids are removed from the fluidized bed while still very hot and are conveyed to ash storage bins by water cooled conveying screws.
Particulate control is still a troublesome issue in fluid bed combustor operation. Adequate particulate separation and collection equipment is required external to the combustor to remove particulates from exhaust combustion gases, and these frequently are quite costly.
An example of a combustor using separate desulfurization and combustion beds is shown in U.S. Pat. No. 4,135,885 issued Jan. 23, 1979, to Alex F. Wormser et al. Sorbent material is applied separately to both beds. In one embodiment, heat transfer from the desulfurization bed is eliminated and sorbent material from this bed is instead passed on to the combustion bed from which it is temporarily withdrawn (together with the sorbent separately applied to this bed) for external storage during periods of turndown in order to conserve heat resident in the sorbent. Heat transfer capacity is significantly diminished in this embodiment, and only limited sorbent transfer between beds occurs.
The cost of a fluidized bed combustor relative to equivalent coal burning facilities which provide an equivalent amount of steam and the same degree of environmental control are about equal. However, more stringent environmental controls and design operating specifications in the fluid bed facility increase the cost of present generation combustors. Increasing the sulfur retention requirements, for example, requires a greater rate of sorbent addition in an AFBC, resulting in greater costs for the sorbent and waste disposal, as well as reduced combustor efficiency due to heat absorption by the in-bed calcination of additional limestone. Particulate carry-over in the combustion gas is also increased by the attrition of the additional sorbent. Sorbent selection is critical in controlling costs, since for good limestones only one fifth to one third of the calcium in the stone combines with sulfur. The remainder of the calcium, initially in the carbonate form, is calcined to the oxide form, absorbing heat.
Most AFBC combustors are currently designed to operate between four and nine feet/second fluidization velocities. These velocities correspond to heat liberation rates of between 280,000 and 640,000 BTU/hour-ft.sup.2. of fluid bed surface area at twenty percent excess air rates. It is obviously desirable to operate at the higher velocities in order to reduce combustor size per unit of energy output. However, at the higher velocities, the following events occur which diminish the performance and increase the cost of facility:
(a) more solids are entrained in the combustion exhaust gas, leading to: increased requirements for particulate control, a reduction in carbon utilization due to the escape of char from the bed as fines before complete burn-out, and increased potential hazard of burning carbon particles penetrating the downstream gas clean-up system; PA1 (b) gas phase residence times in the bed are shortened, leading to poor calcium utilization for sulfur oxide capture. This may be compensated for by using deeper beds to maintain design levels of sulfur oxide control; however, operating with deep beds requires a greater expenditure of power to operate combustion air blowers. PA1 (1) a sorbent preheat-calcination-sulfur oxide capture zone with consequent cooling of the combustion exhaust gas occuring over several stages; PA1 (2) a coal volatiles combustion-sulfur oxide capture zone (which may comprise a single stage); PA1 (3) a coal char combustion-sulfur oxide capture zone (which also may comprise a single stage); and; PA1 (4) an ash cooling-air preheat zone occuring over one or more stages at the bottom of the combustor.
Approximately fifty percent of the thermal energy released to produce steam is absorbed by boiler tubes immersed in the bed. Heat transfer coefficients to the boiler tubes range from thirty to fifty BTU/hour/ft.sup.2 /.degree.F. However, the remaining fifty percent of the energy is removed in the convective banks where heat transfer coefficients range from five to fifteen BTU/hour/ft.sup.2 /.degree.F. This results in approximately twenty-five percent of the tube surface area being immersed in the bed, and the remaining seventy-five percent in the convective bank. Reducing the excess air requirements shifts a greater fracton of the thermal absorption requirements to the bed, where less heat transfer face is required per unit of heat absorbed. The greater quantity of unburned reducing hydrocarbons locally available surrounding a burning coal particle results in reduced NOx generation; however, CO and unburned hydrocarbon emissions increase.
The feeding of fuel to such systems, and the removal of the spent residue therefrom, poses significant problems. Failure to provide for even fuel distribution throughout the combustion bed leads to localized reducing zones in the areas of excess fuel. This causes large temperature gradients and leads to inefficient heat transfer. Further, the reducing zones do not remain wholly stationary on the bed and thus differing portions of the bed are exposed alternately to oxidizing and reducing environments, thus hastening corrosive deterioration. Various kinds of fuel feeders have been used in an attempt to combat this problem, but most of these are quite costly and further increase the significant capital costs of such combustors.
The removal of undesired pollutant gases and particulates in such systems is also difficult and generally costly. Reactant materials such as limestone or dolomite are commonly added to the fluidized bed to remove constituents such as sulfur oxides which result from the combustion process. However, the reaction between these reactants and the combustion products is frequently significantly less than complete. Thus, the unwanted constituents are only partially removed, and a large portion of the reactant material remains unreacted and, to that extent, wasted.
Present designs of AFBC boilers are thus highly constrained; every change in design specification which improves certain aspects in cost and performance normally produces other detrimental cost and performance factors.